How to make tied knots in Blender?
You can start with a simple mesh which roughly defines the knot shape, which for now is flat. Top view here:



Then give it some relief and add a subsurf modifier, still roughly :



Now, convert it to a curve :


Alt+C then curve from mesh
Enter edit mode, set the curve type to bezier then the handles to auto
Now set some bevel to the curve




Doing this, you may have some adjustments to do, but initial vertices (in "mesh mode") are much more easy to manipulate than starting with a curve from the beginning :